This invention relates in general to power switches for connecting accessory devices, such as cellular telephones, to automobile charging circuits via the vehicle cigarette lighter socket.
U.S. Pat. No. 4,733,100 discloses an automatic on/off switch circuit with time delay for powering a radar warning receiver or the like from the cigarette lighter socket of an automobile. The switch is placed in its on condition to electrically connect the radar detector to the automobile charging circuit in response to the electrical noise produced when the automobile engine is running. The switch circuit integrates the noise signals to drive a switching transistor conductive. A predetermined time after shut down of the automobile engine (and disappearance of the noise signals), the switch reverts to its off condition and disconnects the radar detector.
The patented circuit responds to a wide range of noise signals and can be made to respond to noise signals other than those produced by a running engine, e.g. an automobile fan or a clock. Noise generated by devices requiring relatively large currents, such as CB radios or cellular telephones, can be sufficient to cause the patented circuit to falsely trigger in an oscillatory manner, which could result in the automobile battery being discharged. It should be noted that the patented circuit is intended for incorporation into the radar detector proper and therefore only needs to discriminate against a limited range of signals and noises, i.e. those produced by the radar detector (or other particular product).
The present invention contemplates a stand alone switch intended for use with a wide variety of both small and large current accessory devices. Special care is taken to assure that the electronic switch is not falsely triggered by signals and noise produced by any of the types of apparatus with which the switch will be used. The inventive electronic switch is only turned on in response to a sharp drop in B+ voltage, corresponding to energization of the vehicle starter motor, and is maintained in an on condition if a unique signal from the alternator is received within a predetermined time after the turn-on transistor has initiated switch operation. When the automobile engine is turned off, the electronic switch resets after a predetermined time and cannot be reinitiated except by the turn-on transistor. The unique alternator signal that maintains the electronic switch in the on state is the negative half of the alternator ripple voltage. This signal is unique and differs from any other signal present in the automobile environment or in the accessories that would be operated from the switch.